Field of the Invention
The present invention relates to a retardation element, a liquid crystal display device, and a projection display device.
Description of the Related Art
In recent years, an optical compensation technique utilizing a retardation element has been applied to liquid crystal display devices in order to improve a contrast property and a viewing angle property. Examples include black luminance correction in a vertically aligned liquid crystal. In order to correct polarization disturbance due to a pretilt angle of a liquid crystal, birefringence of oblique incident light, or the like, there are considered a method of performing optical compensation by placing a retardation element made of crystal or the like in parallel with a surface of a liquid crystal panel, and a method of performing optical compensation by placing an organic material having a birefringence such as a polymer film in parallel with a surface of a liquid crystal panel (see Japanese Patent Application Laid-Open (JP-A) Nos. 2005-172984 and 2007-101764, and Japanese Patent (JP-B) No. 4566275).
However, in a method of machining a monocrystal as a retardation element, in particular, with a view to performing compensation taking into account even a pretilt angle of a liquid crystal, it is necessary to cut out the monocrystal at a predetermined angle from the crystal axis. This necessitates a very high level precision in cutting, polishing, etc. of the material, and high costs are required to realize such a precision. Furthermore, axis control is not easy with a stretched film or the like.
Hence, there is considered a method of placing a retardation element itself to be inclined from a liquid crystal panel (see JP-A Nos. 2006-11298 and 2009-229804).
However, there is a risk of shortage of space needed for the inclination in projectors that are becoming increasingly smaller in size. Furthermore, there is a problem in durability, with susceptibility to deterioration due to heat and UV rays.
Meanwhile, as a retardation element to which thin film formation by oblique deposition of dielectric materials is applied, there is proposed a retardation compensation element in which a negative C-plate formed of alternately stacked layers of high and low refractive index materials, and an O-plate formed of two or more obliquely deposited films are combined (see JP-A No. 2006-171327). This technique proposes a retardation element that compensates for disturbance in polarization of oblique incident light incident to an optical modulator with the negative C-plate having a structural birefringence based on the alternately stacked layers of high and low refractive index materials, and compensates for disturbance in polarization caused by a pretilt angle with the O-plate formed of two or more obliquely deposited films.
However, it is necessary to stack a total of eighty layers in order to produce a negative C-plate, and it is also necessary to provide an antireflection layer separately, which raises concerns about high costs and a long lead time.
There is also proposed an optical compensation method utilizing two retardation plates formed of obliquely deposited films (see JP-A No. 2009-145863). According to this proposed technique, each retardation plate is rotated in the in-plane direction to be at a position of an optimal related angle, which is expected to improve the contrast.
However, there are concerns about high costs and expansion of the mounting space, because two retardation plates are used and a rotation mechanism is necessary.
There is also proposed a retardation plate that is used with a super twisted nematic (STN) liquid crystal device, and formed of multiple layers of inorganic thin films obliquely deposited from different angular positions that are shifted in the in-plane direction of the substrate, which is the deposition direction, at constant angular intervals by the same degrees as the angle of twist of the STN liquid crystal device (see JP-A No. 2006-171327).
According to this method, it is necessary to perform deposition from multiple angular positions in order to reproduce the twist of the STN liquid crystal device by deposition, which requires special deposition equipment and raises concerns about increase of a lead time due to the multilayer structure. There is also a fundamental problem that this technique cannot be used for other than STN liquid crystal devices.
There is also proposed a liquid crystal display device using a retardation plate including at least two retardation compensation layers arranged such that their layer surfaces face each other, such that they have different retardation values, and such that their optical axes, which correspond to the fast axis or the slow axis of the constituent material of the retardation compensation layers, are directionally varied in the plane of the layer surfaces (see International Publication No. WO2008/081919).
However, this proposed technique requires an adhesive because the two compensation layers are formed by sticking, which raises a problem in heat resistance. This technique also requires two substrates, and raises concerns about high costs.
Hence, it is currently requested to provide a retardation element, a liquid crystal display device, and a projection display device that can effectively and highly precisely compensate for characteristic changes of normal incident light, and characteristic changes of oblique incident light due to the thickness of a liquid crystal layer, can greatly save the installation space, and are also excellent in durability.